The shortage began last fall, when the U.S. Food and Drug Administration (FDA) says "manufacturing delays" caused production to lag.

One of the drug's three U.S. manufacturers, Hospira, said in a statement that its delays were caused when the company was unable to obtain the active ingredient, cytarabine, from its supplier. Hospira and another manufacturer, APP, also had a problem with crystallization in drug vials. APP ended up recalling a supply in February.

Bedford Laboratories, the third manufacturer, said in a statement its delays are "due in part to the fact that Bedford continues to face increased market share of product demand due to a decrease in competitor's capacity."

Cytarabine is used as part of a drug regimen against acute myeloid leukemia (AML), a relatively rare cancer. It's sometimes used to treat acute lymphoblastic leukemia -- ALL for short.

For patients with AML who take cytarabine, the drug is the difference between a shot at life and certain death.

"Since its introduction, we can claim cures in 40 to 50 percent of patients," said Kantarjian. "Without the drug in the treatment regimen, the rate is zero."

Doctors say there are no suitable substitutes for cytarabine, leaving AML patients without a viable alternative. There are other drugs available for treatment of ALL.

Kantarjian was so concerned about the shortage that he emailed thousands of oncologists from all over the country, asking them how they've dealt with the diminished supply of cytarabine.

M.D. Anderson, Kantarjian said, is fortunate because it's a large hospital and was able to purchase a long-term supply of the drug, so there is an adequate supply there right now.

Some Hospitals Turning Patients Away

A number of hospitals said they have to ration supply, meaning some patients won't get the doses they need. It also means they have to turn new patients away.

Nebraska Medical Center in Omaha said, "At one point last week we were completely out of enough cytarabine for any new patients. We had enough to finish the current patients we had in the hospital but no new ones."

Johns Hopkins Hospital in Baltimore said it only had a two-week supply. "We are really worried if we get a new patient with AML -- that will be very hard to keep up with."

Some hospitals said the shortage forces them to reserve whatever cytarabine they have for their current patients.

Dr. Jeff Hord, a hematologist-oncologist at Akron Children's Hospital in Akron, Ohio, said it is treating three or four patients for AML, and its supply of cytarabine is set aside for them.

One of those young patients, two-year-old Payton Maglich, has already had five treatments and has one more to go. Hord said there is enough for that final treatment, but her mother, Shari, still worries.

"When I first heard about the shortage last week, I thought there wouldn't be enough," said Shari Maglich. "I was told not to worry, and the treatment hasn't been cancelled yet. It's in the back of my mind, but I keep thinking positive."

While Payton is fortunate, Hord said he and his colleagues take things day by day.

"I have never seen a situation like the one we're in right now," said Hord. "I can't look much beyond the next treatment."

Cytarabine and Other Drug shortages Concern Doctors

Back in December, ABC News reported that hospitals were facing the biggest drug shortage in decades. Most of the drugs were injectables, and included the potent chemotherapy drug doxorubicin.

In his email to other oncologists, Kantarjian expressed his frustration at the number of drug shortages. According to the American Society of Health-System Pharmacists in Bethesda, Md., there are currently more than 170 drug shortages.

The FDA says there are a number of reasons for the shortages of injectable drugs.

"FDA has continued to see increased numbers over the past several years of shortages involving older sterile injectable drugs. There are several reasons for this including the limited number of firms making these older sterile injectables, capacity constraints at the manufacturers which often preclude ramping up production, and the overall complexity of the manufacturing process for injectable drugs," the agency said.

An editorial in the Journal of the American Medical Association also blames the shortages on a lack of profitability. Many of the drugs are generic, expensive to produce but not profitable to sell.

But the manufacturers of cytarabine say they are committed to replenishing supply, and the FDA says it continues to work with the companies to ensure that happens.

Hospira says it started shipping product last month and plans to ship more this week, Bedford says it expects to release 82,000 vials in the coming weeks and APP says it will start distributing more vials next week.